Posted
by
CmdrTacoon Thursday May 27, 2010 @10:59AM
from the get-where-yer-going dept.

s122604 writes "The [X-51A Waverider]'s scramjet engine accelerated the vehicle to Mach 6, and it flew autonomously for 200 seconds before losing acceleration. At that point the test was terminated. The Air Force said the previous record for a hypersonic scramjet burn was 12 seconds. Joe Vogel, Boeing's director of hypersonics, said, 'This is a new world record and sets the foundation for several hypersonic applications, including access to space, reconnaissance, strike, global reach and commercial transportation.'"

How useful is this in the long run? What was the burn ratio compared to other scramjet vehicles of recent design?

Are there even any other scramjet vehicles in the operational testing phase? I was under the impression that the X-51, and the other vehicles in the Hyper-X program, are the only ones that've actually flown. Scramjets aren't exactly easy to test in the lab.

The temperature of objects produces (from what i recall of physics) black body radiation - meaning it produces light wavelengths. Just because we associate melted iron being red hot, doesn't mean other metals melt when they start to glow. It just means they are hot enough to produce enough black body radiation that we can see. Look at mercury for example as an opposite.

The differences are in how compact the engine is and exhaust velocity. Airliner engines are designed solely for efficiency and as such have bypass ratios that make start to look like a helicopter mounted sideways in a tube. The actual power generating bit of the engine is tiny and most of the thrust comes from shunting air through the outer parts at relatively low speeds without ever being compressed.

Generating exhaust simultaneously at high rate, high velocity and in a compact package is vastly different.

Others have expressed the detail, so I won't be redundant, but the J58 is on the extreme end of Jet technology even today, and that was the point of my analogy.
Those airliner engines you mention produce more thrust and are more economical to operate and to maintain, by a very healthy margin. However, they are only good up to about 500 kts or so, as opposed to the 2000+ kts the J58 is capable of. Nor will they function at all about 60000 feet, whereas the j58 will at full or nearly full thrust. So, in comparison, modern airline engines of which you speak are not in the same class of tech, nor would you expect them to be, since their purposes are far different.
On a side note; it's note related to the tech of the engines themselves, of course, but those airline engines also will never push as pretty an airframe through the air, wich disqualifies them on the asthetic front too.... {smile}

I agree that using the J58 as an example of a typical jet engine is rather like using an atom bomb as an example of an explosion.At the same time, since the J58 is essentially a turbojet/ramjet hybrid, it might be said to be the distant forefather of the X51 engine.

Oh, and anything that makes me go look at pictures of the most beautiful aircraft in the world, the SR-71, is a good thing.

Yes, but not scramjet technology that uses ordinary jet fuel to power the engine. Said scramjets used hydrogen instead, and can't maintain flight for long because of how bulky the large hydrogen tanks are.

I think you misunderstand his point (or perhaps I am giving him too much credit for asking a really good question). Mach speeds are a ratio of the speed of the aircraft to the speed of sound. Mach 6 therefore means "six times the speed of sound". OK, nothing difficult there; most people here probably knew that already. Here's the rub: what is the speed of sound? Hint: it's not the same at sea level and at the 0.1Mm you mention, because the speed of sound varies with the density of the atmosphere. In other words, Mach 6 at sea level (~4500 mph) [answers.com] is not the same speed as Mach 6 at, say, 100,000 feet above sea level (~4100 mph) [answers.com].

The usefulness, overlooked in the summary and (brief) article, but reported in The Register [theregister.co.uk] (longer article), is that this vehicle used jet fuel (JP-7) instead of Hydrogen. Additionally, it apparently flamed out at Mach 5, not 6.

The hypersonic X-51A ignited, burning a mixture of ethylene and JP-7 jet fuel, and once well alight switched over to all jet fuel operation.

Useful??? They had to use a B-52 to get into the air. Then a rocket booster to go to 4.5Mach. Then this precious little stove pipe took it to 5.0 Mach.

Yeah! How can you call that useful? And speaking of wastes of time during our war with the Japs and the Nazis, did you hear what those crazy eggheads over at Los Alamos are doing? They're trying to build a "nuclear bomb" -- but all they've managed to do so far is irradiate a bunch of scientists. How is that useful?

Why are these engines burning for such short times? Are these engines so early in development that they really can't get them to be stable and safe for more than 12 seconds? Sounds a lot like fusion: it works but it's not yet useful.

Likewise you want to stop the test before failure so you can look for signs of component wear and material stress so that you know what to improve for next time. Stopping at 200 seconds and finding this out is very useful. Stopping at 201 seconds after it has exploded and you have to work out from the pieces what went wrong is not as informative.

Likewise you want to stop the test before failure so you can look for signs of component wear and material stress so that you know what to improve for next time. Stopping at 200 seconds and finding this out is very useful. Stopping at 201 seconds after it has exploded and you have to work out from the pieces what went wrong is not as informative.

Since the vehicle was deliberately crashed into the ocean and not recovered - there's nothing to examine for wear and stress, whole or in pieces.

Not sure how fast it reached Mach 6, however, at Mach 6 in 200 seconds, you have covered over 250 miles (400 km). That's a lot of ground. No point continuing the experiment if whatever you are monitoring can't be monitored because its so far out.

Scramjets have no moving parts; it's a duct and a fuel injector. That's it. Wear is a non-issue here.

Figuring out what shape that duct needs to be in order to get stable combustion, however, is far from a trivial problem; it's not just the fluid dynamics of the supersonic flow that must be considered... thermodynamic losses can be large enough to quench the flame, and where in the duct those losses occur is as important as their magnitude. With most sorts of engines, second-law analysis is only something you look at to refine the efficiency of the technology once it's mature and functional. With scramjets, it's critical to making them work at all. Building one of these, with all our modern computer modeling technology, is kind of akin to building the first rockets with no computers at all.

Not only that, but you can't always be sure which part is the weak link, what will break. A few tests at hypersonic is guaranteed to make an engineer say "ok, that's an issue we weren't aware of", and confirm much of what they already knew. They may have to make some part that they *thought* would be ok out of a different, stronger alloy, etc. Of course, at this speed, every tiny error in engineering is amplified, as at Mach 6, you cross a lot of real estate in just a few seconds, so the word "precision" doesn't adequately describe the level of perfection required in the test system build.

Being a pioneer at anything guarantees surprises, and best of all, learning new things.

Keep in mind that at mach six 200 seconds is 400 kilometers. That's already enough range to make a useful weapon (and yes I realize there was some acceleration time in there). Heck, that's already longer range than the most advanced missiles that many countries have. Increase the stability to just 10 minutes of burn time and you've got a missile that can go 5% of the way around the world.

According to this article [wikipedia.org], ICBMs already travel at close to Mach 20 during the boost phase and Mach 10 on impact. Missiles have no problem going faster than jets, why do you think they are so effective?

ICBMs do not do any jetting at all, its not a comparison, as they are ballistic rockets. You can think of them as flying mortars more than missiles. The closest applicable missile would be a cruise missile, but honestly, they are designed more for distance than speed. You don't need your cruise missile to get there ridiculously fast, but you do want a ridiculous range for them, at a sufficient speed to not be destroyed or give advanced warning, while still making it to the target in time for intercept.

It's all part of the Prompt Global Strike program. Eventually the goal is to have scramjet powered missiles with a range of 12000+ miles. Allowing you to fire an obviously non-nuclear missile from Kansas to North Korea and have it hit the target in less than 3 hours. Basically, even launching a B2 for a direct strike takes days or weeks. Refueling planes need to be put into the air, mission plans drawn up, clearance over foreign airspace cleared, pilots briefed, etc. They want to be able to say "Fire" and have the missile in the air within minutes and at target within hours; with the added benefit of not putting any American lives in harms way.

Because Wilbur already had the first chance, Orville took his turn at the controls. His first flight lasted 12 seconds for a total distance of 120 feet (36.5 m) - shorter than the wingspan of a Boeing 707.

It is a question of stages. The timescale that need to be designed for initially are based on the engine size divided by gas flow speed. That is maybe 10s of microseconds. The next timescale may have to do with longer scale oscillations in the engine structure...that is probably on the order of 1 - 100 milliseconds (10-1000 Hz). After that, you need to start worrying about heat...things melting, expanding, coating eroding...those are probably 1-100 seconds.

Testing scramjets on the ground is really, really hard, and you can only do it for a very short time (much less than a second).

Testing scramjets in flight is really, really expensive. And when your funds are limited, you can only build subscale air-dropped missile-sized vehicles instead of full-sized, self-launching, reusable ones (in part because of the "cheaper now and more expensive long-term" being prefereable to "more expensive now and cheaper in the long run" thinking that brought us

Mach 6 during 12 seconds is 23 kms. During 200 seconds it is 790 km. Useless for a plane, but imagine what a drone or a missile could do. Short range but no possibility of missile interception : it is already useful despite being a prototype.
I am however disappointed : I thought it was supposed to achieve Mach 10 ?

From the FX claim Sorb [ideosphere.com] currently trading at a 10% chance of coming true:

Suborbital transportation will exceed high-mach air transportation by the year 2020. "Suborbital" means any high-mach, non-orbital flight where the majority of the distance is covered without benefit of locally available gasses as the primary propulsion reaction mass. "High-mach" means the majority of the distance is covered at a speed of mach 2.5 or greater. "Non-orbital" means the total flight path distance is less than the circumfren

It's entirely possible to do orbital velocities and not be in orbit... It tends to end up in being turned into smush, and/or being propelled out the atmosphere at some point, but that doesn't change the fact that it's possible.

There's no fundamental reason why you couldn't orbit within the atmosphere at Mach 22, with the occupants experiencing weightlessness. We don't currently have the materials and engines to withstand such conditions, nor would it make any sense to try, but it could be done in theory.

Anyway back to the OP's point, since kinetic energy is proportional to the square of the velocity, Mach 6 is only 7% of the energy needed to reach orbit. IMO, scramjets are just a complex diversion if the goal is to go into orbit.

Crap. And I just bought the aftermarket exhaust cans for my personal scramjet, since they were advertised to have a way cooler than stock exhaust sound. At least you'll still be able to see the underbody lights I added as I zoom past...

That brings up another point; I noticed that one of the applications they're pinning on this technology is for orbital launches. Being still a jet this engine requires air, so I'm pretty safe in assuming the application is for an assisted orbital target or as some kind of launch platform, right?

"In its first flight attempt, the Boeing [NYSE: BA] X-51A WaveRider today successfully completed the longest supersonic combustion ramjet-powered flight in history -- nearly three and a half minutes at a top speed of Mach 5."

My understanding is that it didn't reach the 300 seconds Mach 6 burn it was hoping for. 200 seconds and Mach 5 isn't all that bad though...

Jet engines still have propellers to compress air, into which the propellant is injected where this mixture then ignites.

Scramjets just use their immense speeds to do the compression with a funnel, but that also means they need more conventional means to reach these high speeds (such as getting a ride on a B52 plus gravity)

Questions

What is the fuel efficiency per kilometer traveled?

Can a scramjet reach escape velocity, or get close enough so that additional oxygen for the fuel doesn't make up more than half the payload?

Did they find a new way to lose heat at supersonic speeds? Otherwise these rides will remain short ones.

If it is capable of propelling whatever it's carrying to 11.2 km/s (just under Mach 33 at the Earth's surface). Since this one only goes to 6, there's still a ways to go.

Just as a foot note - if you're travelling at 11.2 km/s in a circle with a 6,378 km radius (Earth's equator), you will experience an acceleration opposite to gravity of 19.67 m/s^2 (aka 2 g). Just slightly slower if you do it at an altitude of 10 km (19.64 m/s^2). At the altitude this one reached, you'd hit 19.6 m/s^2

With regards to question 3, they have developed some pretty tricky ways to help cool the engines and the body of the vehicle. For instance, they cool the engines by circulating the JP-7 fuel through the body to absorb some of the heat to help bring it to the combustion point. This is very similar to how rocket nozzles are currently cooled to slow melting/failure:

The scramjet will circulate the fuel behind engine walls to cool the structures. Without such active cooling, the temperatures in a scramjet could reach 5,000 deg. Fahrenheit, high enough to melt virtually any metal on Earth. Solving the cooling challenge is a major AFRL/Pratt & Whitney achievement.

My wager is that the entire vehicle took thermal control into its design considerations and it uses a combination of geometry, aerodynamics, and fuel management to help sink heat at an appropriately high rate to prevent too much for a build up. However, since I don't have the design specs, and I doubt anyone outside of the military will, for awhile at least, I can only speculate. You also have to understand that at those speeds, your gas dynamics become a problem of rarified gasses [wikipedia.org] and heat management becomes a very tricky problem indeed, one that can't be approached by traditional cooling means. So in summation I would guess yes, they have probably found some very cool new ways to sink heat at hypersonic speeds.

Atmosphere isn't just compression, it's reaction mass. The compression creates the conditions for acceleration when the fuel is burned, but it's heating of the compressed air that causes the high force that accelerates the air backwards, accelerating the vehicle forwards.

You can bring your own oxygen, but it's just going to make a pretty flame coming out both sides of the combustion chamber unless there's mass flowing into the inlet and being pushed out through the nozzle.

I said this above already, but what you do here is you use a first-stage rocket motor to get up to scram-jetting speeds. Then the second-stage motor gets you to a certain point in the atmosphere where it loses thrust. Then you light your third stage.

This makes sense only if the specific impulse (look it up) from the scramjet exceeds that from a rocket motor, or it's ridiculously cheap and still gets the job done. I'm guessing it's not ridiculously cheap. But given that you don't have to bring the reaction mass for the scramjet with you, it might be more efficient than a rocket of the same mass. Meaning you can omit a heavy rocket motor and use a lighter scramjet and put the saved mass into the payload.

When do we get a combination turbojet/ramjet/scramjet? Or will we be launching aircraft piggyback (or underwing) for the foreseeable future? This seems like a great technology for amazingly pissed-off artillery shells, I can imagine a ramjet that turns into a scramjet pretty easily if it doesn't have to turn back.

Commercial applications do usually follow. Whether or not you agree with it, military research has led to an enormous number of scientific advances that were initially used by the military but later disseminated more broadly. Jet engines, the Internet, cryptography, GPS, nuclear reactors, etc. Mach 6 might be inefficient overkill for Earth-side transportation, but it may provide a viable means of launching spaceflights one day.

Whether or not you agree with it, military research has led to an enormous number of scientific advances that were initially used by the military but later disseminated more broadly. Jet engines, the Internet, cryptography, GPS, nuclear reactors, etc

My Google-Fu seems to be failing at the moment, but wasn't the internet originally conceived to keep track of nuclear weapons?

Are you trying to point out that the internet wasn't a military innovation by stating its purpose was to track nuclear weapons (*military* nuclear weapons)?

And for what it's worth, the original purpose was to allow communication between points with no single path of failure (insert beneficial military application here like giving combat orders in the event of a nuclear strike); it started in universities, national labs, and large military bases who had the budget to pull the wires, and before we knew it there were all kinds of fun uses for it like MUD games and e-mail and slashdot and finally facebook; the ultimate military weapon.